Light weight composites which have a good balance of high stiffness, high toughness, and low weight are used in many applications which require low flexibility and can benefit by reduced part weight. Transportation is one industry which has a need for such materials, for example, as a component of a vehicle or for an object (such as a container) which is being transported.
The oil crisis in the late 1970s and early 1980s prompted development of low weight laminates of metal sheets and polymers. Some of the these efforts are reviewed by Kim at al. (Jang-Kyo Kim and Tong-Xi Yu, “Forming and Failure Behavior of Coated, Laminated and Sandwiched Sheet Metal: A Review”, J. Mater. Process. Technology 63:33-42, 1997). In their review, Kim et al. describe the use of composites containing steel faces and either nylon 6 or polypropylene cores of thickness of 0.2/0.4/0.2 mm for automotive body panels. This kind of sandwich has exceptional high flexural strength to weight ratio thus leading also to its successful implementation in many engineering applications (Mohr and Straza, 2005). However, specific technical problems must be solved when thin sandwich sheets for the automotive industry are concerned. These problems are related to what is required to adapt such sandwiches to mass production, especially low cost forming processes, such as the already existing stamping processes (and particularly using existing stamping equipment). The ability to use standard stamping lines will reduce capital costs, increase versatility of existing equipment, reduce transition and implementation times, and circumvent major difficulties related to unconventional sandwich-specific manufacturing techniques.
As far as the development of very thin sandwich sheets is concerned, two major approaches have been pursued. The first approach is a group of all metal sandwiches of thin metal skins and a metallic cellular core. Generally, this group can be welded similarly to a typical weldable metal sheet, since it tends to conduct electricity. The second approach, however, typically has two faces of thin metal sheets, separated by a central polymer core layer (which typically are relatively soft viscoelastic materials), and is structured so that it does not conduct electricity across the core layer largely due to the insulative and non-conductive nature of the polymer core layer. The polymeric material in any core of this structure may in fact be an insulator. Thus such sandwich sheet may not be joined by spot welding or require welding conditions (e.g., force, current, cycles, weld time) that may be substantially greater than required for sheet metal having the same thickness. When weldability is concerned, the all metallic sandwich is, therefore, preferred.
Gissinger et al. (1994, U.S. Pat. No. 5,347,099) discloses a method using a specific arrangement of rollers and partially overlapping sandwich sheets as a possible approach to facilitate welding.
Straza (International Patent Application Publication No. WO2007/062061) discloses a method of manufacturing a metal core sandwich structure of a cellular metal core having a shape selected from a group consisting of: octagons, hexagons, pentagons, squares, rectangles, triangles, and circles. Clyne et al. (2004, U.S. Pat. No. 6,764,772) describe a sandwich material of two metal plates which are affixed to and separated by a fibrous metal core that is generally exposed to air, wherein substantially all of the fibers are inclined at an acute angle to the plates. However, one possible difficulty with those specific cellular metal cores is that the structure is not continuous and thus the face sheet is not supported over the length of the cell. In case of soft face sheets, the thin cell walls may be damaged locally. Their applications thus tend to be limited. It is also a costly structure in that it generally requires the use of expensive materials for resisting corrosion.
Typically, the formability of some metal composites has been found to be inferior to corresponding homogeneous sheet metal of the same thickness. The composites have limited drawing ratios and a higher tendency to wrinkle as well as several potential geometrical defects in bending. For some materials, these defects may be the result of large shear deformations in the interlayer because the core material is weak compared to the sheet metal. Another possible vulnerability of many composites (e.g. sandwiches or laminates), may be their susceptibility to dents. Wrinkling may be attributed to low yield strength of the core material.
Kim et al. (2003) tested the formability of a certain Aluminum/Polypropylene/Aluminum sandwich sheets as a possible material for automotive usage. Their analysis suggested that certain polypropylene cores may result in sandwich sheets having improved formability.
Efforts in the art to modify polymeric cores of sandwich composites by introducing a fibrous phase generally have produced the effect of restricting the elongation and thus decreasing the ductility of the composite material. Accordingly, such materials have not been given considerable attention for a stampable composite.
Efforts in the art to enhance welding of sandwich composites generally have been directed at modifying polymeric cores of the sandwich composites by loading them with relatively large amounts of conductive particle fillers.
Thus, there still exists a need for light weight composite materials which have improved formability over the existing materials. As such, there is a need for sandwich sheets or laminates which have improved ductility so that low-cost standard sheet metal forming technology could be employed.
Also, there continues to exist a need for a weldable light weight composite having a polymeric layer that does not impede weldability. The ability to join a composite part to other metal containing parts, in particular by welding (e.g., by a resistance welding technique, such as spot welding) is highly desirable.
Additionally, there is a need for light weight composites that can be processed to form an outer covering layer (e.g., to form one or both of a decorative covering, or functional covering, such as a functional coating to improve bonding of the surface to another material such as an adhesive).